THE DISTRIBUTION OF ALTERATION AND MINERALIZATION ASSEMBLAGES OF THE MINERAL PARK MINE, MOHAVE COUNTY, ARIZONA

Abstract

The Mineral Park mine is a porphyry copper-molybdenum deposit developed within and adjacent to a Laramide quartz monzonite porphyry stock which intrudes Precambrian rocks in northwestern Arizona. The Precambrain sequence consists of older, broadly folded metasedimentary and metavolcanic rocks which were intruded by a 1700-1800 m.y. old granite gneiss batholith. The contact between the two Precambrian terranes is a major structural element in the district and appears to have been important in localizing the Laramide intrusions and mineralization. Alteration is defined by early pervasive biotitization of hornblende in the Precambrain rocks and by recrystallization of rock biotite in the quartz monzonite porphyry. Pervasively biotitized rocks are crosscut first by biotite and then by K-feldspar veinlets. Fracture-controlled, economic sulfide mineralization then began with quartz-molybdenite-K-feldspar-anhydrite and was followed by quartz-chalcopyrite-K-feldspar-anhydrite. This potassic alteration and accompanying mineralization occur throughout the deposit and are crosscut by later quartz-pyrite-sericite veinlets. Orientations of mineralized fractures evolved through time from EW during molybdenum mineralization to NW during quartz-pyrite-sericite mineralization. Fracture densities during molybdenum mineralization averaged 0.05 cm⁻¹ and increased to 0.14 cm⁻¹ during quartz-pyrite-sericite mineralization. Sulfides were deposited from low salinity fluids (0.5 - 2.0 molal) in the temperature range 330°-360°C. High salinity fluids occurred only with quartz that was earlier than sulfide deposition. No homogenization temperatures greater than 440°C were observed. Molybdenum mineralization cuts all rock types and defines a vertical cylinder with a distinct low-grade core. Ore grade molybdenum mineralization is equally distributed between Laramide and Precambrian rocks, and overall grade decreases with depth. Hypogene copper mineralization has a greater lateral distribution than molybdenum mineralization, and surrounds a low grade core coincident with the low grade molybdenum core. The distribution of alteration and mineralization assemblages and the fact that both of these features crosscut all exposed rock types suggest that copper-molybdenum mineralization was not temporally related to the quartz monzonite porphyry exposed in the mine area. The narrow range of homogenization temperatures observed and the lack of high homogenization temperatures compared with the results of computer modelled systems indicate formation of mineralization 2 to 3 km above a source intrusion. Because no evidence for boiling was observed, only minimum pressures of formation can be determined. Minimum pressures during sulfide deposition varied from 180 to 80 bars. These pressures correspond to minimum depths of formation of 2 to 3 km which is in good agreement with an inferred depth of burial of approximately 3 km based on stratigraphy restored from the adjacent Colorado Plateau.